The present invention relates to holographic optical devices, and particularly to devices which include a plurality of holographic optical elements (HOEs) carried by a common light-transmissive substrate. The invention is capable of being implemented in a large number of applications. Described below, for purposes of example, are the following implementations: division multiplexing/ demultiplexing systems; compact holographic displays; compact holographic beam expanders and compressors; and holographic visor or head-up displays.
Recently, there have been significant advances in optical fibers technology for telecommunication systems. One of the proposed methods to exploit more efficiently the high potential bandwidth of optical fibers is by wavelength division multiplexing (WDM). With this technique, a large number of communication channels can be transmitted simultaneously over a single fiber. During the last decade, various systems for implementing WDM have been proposed, including systems based on birefringent materials, surface relief gratings, Mach-Zender interferometry, and waveguides. Unfortunately, these proposed systems generally suffer from low efficiencies or from a strict limitation on the number of channels.
Another proposed approach is to use a thick reflection hologram as described in N.Moslehi, P.Harvey, J.Ng and T.Jannson, Opt. Lett. 14,(1989) 1088. However, the necessity to use a conventional aspheric lens for collimating and/or focusing the light waves makes the system bulky and space consuming. Furthermore, a single holographic element is very sensitive to the signal's wavelength which usually depends strongly on temperature. One application of the present invention described below enables wavelength division multiplexers/demultiplexers to be constructed having advantages in the above respects.
The invention also enables improved holographic displays to be constructed. Since its inception three decades ago, there has been significant progress in the area of display holography. Indeed, it has become so popular as to play an important role in advertising, packaging and even in art. Yet, the current form of display holograms has some severe drawbacks. The necessity to use a readout light source that must be located at some distance from the hologram, in order to illuminate its whole surface, makes the holographic display systems bulky, space consuming, and sometimes inconvenient to use. Another drawback is that the transmitted part of the readout wave, which is not diffracted by the holograms, usually bothers the observer.
Recently, there have been several proposals, based on edge-illuminated holograms for constructing compact displays that overcome the above drawbacks. The salient feature of these proposals is to reconstruct the holograms with a readout wave which enters the hologram substrate through a polished edge so as to reach the emulsion at a large angle of incidence. Unfortunately, the necessity to enter the readout wave through the hologram's edge introduces some problems, such as the need for a special cover plate (thick, with polished edges), and the requirement of very accurate alignment of the readout wave. Also, the edge-illuminated holograms are very sensitive to the readout wavelength, so the hologram must be reconstructed with purely monochromatic light.
Another important application of the present invention is in providing a compact holographic display which has advantages in the above respects.
A further application of the invention is to provide a compact beam expander or compressor. Beam expanders for magnifying a narrow collimated beam into a beam of larger diameter are typically comprised of a telescopic assembly of two lenses along a common axis and having a common focal point. When used with polychromatic light, e.g., while light, refractive lenses are generally needed. However, with monochromatic light, such as from lasers or other monochromatic sources, it may be advantageous to exploit holographic lenses. The present invention provides a beam expander or compressor which can be used with both monochromatic light as well as polychromatic light.
Another important application for holographic optical elements (HOEs) is in visor displays. There a HOE serves as an imaging lens and a combiner, where a two-dimensional quasi-monochromatic display is imaged to infinity and reflected into an observer's eye. The display can be derived either directly from a CRT or indirectly by means of a relay lens or an optical fiber bundle. Typically, the display is comprised of an array of points whose geometry at readout differs from that at recording. As a result, the imaged array contains aberrations that decrease the image quality. In addition, it is often necessary to record the HOEs at a wavelength that differs from the readout wavelength. This is particularly true when the read-out wavelength is not suitable for recording the HOES. Such a wavelength shift introduces additional aberrations.
Another problem, which is usually common to all types of diffractive optical elements, is their relatively high chromatic dispersion. This is a major drawback in applications where the light source is a CRT which is not purely monochromatic.
Recently several new designs were proposed for improving the performance of holographic lenses. These designs, which only deal with single HOEs, compensate for the geometric and chromatic aberrations by using nonspherical waves rather than simple spherical waves for recording. However, they do not overcome the chromatic dispersion problem.
The present invention may also be used for designing and recording HOEs for visor displays in which both the aberrations and chromatic dispersions are minimized. It is particularly advantageous because it is very compact and can be readily incorporated even into optical systems that have specialized configurations.